Sometimes at checking out carts from Prosociate to Amazon the cart will be empty. Often this is a result of caching plugins.
Your site’s checkout page should not be cached. Why? Because the information on that page needs to change frequently. Adding items to the cart, removing them, calculating cart totals, and all other functions of the checkout page need to reflect the actual state of the page data every time it is loaded. Otherwise, you can (and most likely will) receive unexpected results when it’s time to checkout.
If you are using a plugin such as W3 Total Cache, WP Super Cache, or another similar plugin, you must configure it to avoid caching on the checkout page.